When you're flipping through an origami book, you'll often see instructions that specify you make a valley fold or a mountain fold during a particular step.

Valley folds get their name because they seem to sink back into the paper like a valley. Mountain folds get their name because they appear to raise up from the paper like a mountain.

A valley fold is made by folding the bottom edge of the paper up towards you and then down so it lays over the top of the surface of the paper.

Most people make a mountain fold by turning the paper over, making a valley fold, and then flipping the paper back to its original position. This tends to be easier than trying to fold the paper behind and away from you.

The origami picture frame is a good project to illustrate the difference between valley folds and mountain folds.

To make an origami picture frame:

1. Start with the patterned or colored side of your paper face down. Fold your paper in half vertically, unfold, and then fold it in half horizontally. Unfold to reveal a cross shaped crease pattern. These folds are all valley folds.
2. Fold up the left and right edges 1/2 of the way to the middle crease. Fold the top and bottom edges up the same distance. Again, these folds are all valley folds.
3. Flip your paper over. Fold each of your corners into the center of the paper. These folds are mountain folds. As an experiment, try folding this model without flipping the paper over to see how much more difficult it is to fold the paper behind you into a neat crease.
4. Flip your model over once more and add the desired photo. Your frame can be propped up on your bookshelf on its own or as part of a display of frames made from different sizes of paper.